This invention pertains to method and apparatus for testing the light reflective efficiency of retrodirective optical prisms.
The invention pertains particularly to method and apparatus for the testing and quality control of retrodirective or cube corner reflectors, referred to in the art, and herein, as "prisms". Such reflectors are necessary components of land survey apparatus and distance measuring apparatus, and are used as well in other geodetic applications. Their principal characteristic is that they return a beam of light in its initial path.
However, the degree to which this is accomplished clearly depends upon the precision and accuracy with which the prisms are manufactured. Manufacturers and distributors of such prisms therefore use various means to make sure that the prisms they make, receive, or sell, meet certain criteria. In short, they employ various means for quality control.
Foremost among these is testing by interferometry. Interferometry is widely considered to be a very precise and most accurate measurement tool. In particular, the analysis of interference effects (interference fringes) can be used to determine the correct shape of the prisms. It also can be used to indicate deviations, if any, from true right angles, inhomogeneities within the glass, and other dimensional and optical defects. Some manufacturers even supply interferograms with the prisms they sell, and a number of publications are concerned with the generation and interpretation of fringes that are produced by optical interference.
A general overview, and a more detailed discussion of interferometry and photometry, is found in Jurgen R. Meyer-Arendt, "Introduction to Classical and Modern Optics," Prentice-Hall, Inc., Englewood Cliffs, New Jersey 1972, 4th printing 1978. For a detailed mathematical analysis of cube corner retrodirective reflectors see Edson R. Peck, "Theory of the Corner-Cube Interferometer", Journal of the Optical Society of America, Volume 38, pages 1015-1024 (1948). Interferometry as a rapid method for testing retrodirective prisms has been described by David A. Thomas and J. C. Wyant, "Determination of the Dihedral Angle Errors of a Corner Cube from its Twyman-Green Interferogram", Journal of the Optical Society of America, Volume 67, pages 467-472 (1977).
Practical experience has taught, however, that in actual use in the field some retrodirective prisms perform very well while others do not, and that these differences in performance are not necessarily related to the interference patterns of the prisms. Some prisms have deficiencies that cannot be detected by interferometry. Examples of such defects are dust on the prism surface, weathering and other causes of dullness of the glass, frustrated internal reflection (in non-metallized prisms), defective reflective coatings (in coated prisms), and absorption within the glass. Most importantly, it is the strength of the return signal that determines the practical use value of retrodirective prisms, not the results of interferometry as such.
Accordingly, it is the general object of the present invention to provide a practical, accurate, easily carried out method for the testing and evaluation of retrodirective optical prisms by measuring the intensity of a light signal reflected therefrom and by comparing that intensity with the intensity of a reference beam.
It is another principal object of the present invention to provide simple, accurate, and easily operated apparatus for carrying out the test method.
The foregoing and other objects of this invention are accomplished by the practice of a method which, broadly considered comprises generating two substantially coaxial beams of light, a signal beam and a reference beam. The beams are passed through an optical beam splitter which is adjusted at a predetermined angle to the axis of the beams, i.e. at an angle of .+-.10.degree. preferably .+-.5.degree.. This separates the beams.
The separated signal beam then is passed into the prism to be tested. It is reflected out of the prism parallel to its initial path, and back through the beam splitter. The intensity of the reflected beam is measured and preferably compared with the intensity of the separated reference beam. This gives a measure of the amount of light reflected by the prism and accordingly of its efficiency.
The separated reference beam is either absorbed, or its intensity measured and used as a comparison standard.
Broadly considered, the apparatus of the present invention comprises means for accomplishing the functions above indicated.